The present invention relates to a scanning electron microscope, and particularly to a scanning electron microscope that allows efficient observation of a magnified sample image (high-magnification image) and a whole sample image (low-magnification image).
In a scanning electron microscope, the object lens is conventionally used at a very short focal distance to obtain scanned images of higher resolution, as is typified, for example, by the in-lens system in which a scanned image is obtained by placing the sample between the magnetic poles of the object lens. When the object lens is used at a short focal distance and sample observation is to be performed under high magnification, there is employed a lens control method for scanning by a primary electron beam wherein deflecting coils for the scanning by the primary electron beam are arranged in two stages along the optical axis, and the deflection point of the primary electron beam is set to be in the proximity of the principal plane of the object lens. The above arrangements are provided in order to prevent distortion caused by the object lens or an increase in the beam diameter of the primary electron beam on the periphery of the scanning region. The lens control method described above is called high-magnification mode. On the other hand, when a view search under low magnification is to be performed at a stage before proceeding to high-magnification observation as described above, or the whole image of the sample is to be observed under low magnification, the following lens control method is employed to enable scanning of a wide region (low-magnification state) by a primary electron beam. That is, the exciting current of the object lens is set to be zero or in a weak excitation state, and the scanning of the sample by a primary electron beam is performed by using a one-stage deflecting coil or two-stage deflecting coils wherein the distance between the deflection point and the surface of the sample is set to be longer than that of high-magnification mode. This lens control method is called low-magnification mode. Thus, observation in a wide magnification range from high to low magnification has been made possible by switching between two magnification modes depending on the observation magnification.
Now, in the X-ray ananlysis of the sample by means of a scanning electron microscope, X-rays occurring from within the scanning range of the primary electron beam (view) are detected to identify the constituent elements of the sample in the view by using an X-ray spectrum and collect information on how the constituent elements are distributed in the view (X-ray mapping image) and the like. An X-ray mapping image, in particular, requires not only local element mapping of the sample through high-magnification observation but also general element mapping of the sample through low-magnification observation.
In the case of X-ray observation by using a scanning electron microscope, if an X-ray detector can be placed in the proximity of the sample, the extraction angle of X-rays can be increased, and therefore highly efficient X-ray analysis is performed. However, if a reflected electron that occurs from the sample by irradiation with the primary electron beam as in the case of X-rays falls on the detection plane of the X-ray detector, it may cause an error or a failure. This problem needs to be avoided.
If the sample is placed within the magnetic field of the object lens, as in the case of the in-lens system, X-ray analysis can be performed in high-magnification mode where the object lens is used in a strongly excitated state. This is because the magnetic field of the object lens causes the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. However, in low-magnification mode where the exciting current of the object lens is set to be zero or in a weak excitation state, it is not possible to generate an object lens magnetic field strong enough to cause the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. Therefore X-ray analysis is difficult to perform in this case.
Methods for performing X-ray observation in low-magnification mode include a method in which the X-ray detector is moved so as to keep away from the trajectory of the reflected electron and a method in which a magnet is placed on the detection plane of the X-ray detector to cause the trajectory of the reflected electron to go away from the detection plane of the X-ray detector. However, it is difficult to adopt such methods in the in-lens system because of its structure.